1. Field of the Invention
The present invention relates to a multicolor image forming apparatus and, more particularly, to a multicolor image forming apparatus suitable for image formation by electrophotography.
2. Description of the Prior Art
In recent years, multicolor image forming apparatuses which obtain a full-color copy using a full-color original image (original) are available. A multicolor image allows reproduction from an original such as a portrait, still life, landscape, or the like and also allows to record a large number of data in a single recording image. For this reason, a multicolor image is very suitable for graphs, tables, and the like.
Under these circumstances, a variety of multicolor image forming methods and apparatuses have been developed.
For example, a plurality of latent image forming means and a plurality of developing means are disposed around a rotary photosensitive drum (image carrier). Latent image formation and development are repeated to overlay visible images (toner images) of different colors on the photosensitive drum. Thereafter, the toner images are simultaneously transferred to a transfer material. This method is disclosed in Japanese Patent Laid-Open (Kokai) Nos. 52-106743, 56-144452, 58-79261, and 61-170754.
In a method disclosed in Japanese Patent Laid-Open (Kokai) Nos. 60-76766, 60-95456, and 61-170754, one latent image forming means and a plurality of developing means are disposed around the rotary photosensitive drum, and latent image formation and development for one color are performed during single rotation of the photosensitive drum. Thus, the photosensitive body is rotated a plurality of times to form a multicolor visual image thereon. Thereafter, the multicolor image is simultaneously transferred to a transfer material.
In the former method, when colors to be reproduced are full colors, i.e., yellow, magenta, cyan, and black (if necessary), the latent image forming means and the developing means corresponding in number to the colors must be arranged around the photosensitive drum. Thus, the diameter of the photosensitive drum must be increased, resulting in a bulky apparatus. Control and an apparatus for maintaining very high write precision of the latent image forming means such as a laser, LED, LCS, and the like must be prepared in order to guarantee registration of color separated latent images during formation of latent images. Meanwhile, since a single read-scan operation need only be performed, registration upon reading is good. However, an image memory having a large capacity is necessary.
In the latter method, since only one latent image forming means is required, the apparatus can be made more compact than that in the former method. In addition, since the latent image forming means is commonly used, registration of latent images can be guaranteed. In the latter method, a multicolor image is formed on the basis of the principle shown in FIGS. 1 to 4. Note that FIGS. 1 to 4 show operations until a second development process is performed, and the following operation is the same and is omitted.
FIG. 1 shows a case wherein a latent image is formed by a latent image forming method in which a non-exposed portion serves as a coloring portion, and toner charged in an opposite polarity to the latent image is attached to the latent image, thereby developing the image. In this method, first uniform charging is performed as shown in FIG. 1(b) on the surface of an image carrier in an initial state at a potential of 0 V shown in FIG. 1(a). First image exposure shown in FIG. 1(c) is performed on the charged surface based on color image data of the first color, so that a latent image is formed to have a potential of substantially 0 V excluding the coloring portion. Then, first development shown in FIG. 1(d) is performed for the latent image using a toner T which is colored in a predetermined color and is charged in an opposite polarity to the latent image.
Second uniform charging shown in FIG. 1(e) is performed on the image carrier on which the toner image of the first color is formed. Second image exposure shown in FIG. 1(f) is performed on the charged surface based on color image data of the second color different from the first color, so that a latent image is formed to have a potential of substantially 0 V excluding the coloring portion. The obtained latent image is subjected to second development using a corresponding color toner T'. As a result, a two-color toner image is formed by the toners T and T' on the image carrier.
Similarly, third and fourth latent image formation and development operations are performed to form a multicolor toner image by overlaying color toner images.
In the case of FIG. 1, since the latent image is developed using toner charged in an opposite polarity to the latent image, a developing density of each color can be easily increased. Therefore, a clear multicolor image can be easily formed. Note that a potential may be left on a toner image formed earlier and color mixture tends to occur. Thus, in order to prevent color mixture, a DC developing bias is gradually increased for the following development processes, and a charging potential is correspondingly increased. When the obtained multicolor toner image is transferred to a transfer material, corona transfer can be performed by means for uniforming a charged polarity of toners.
FIGS. 2 to 4 show cases wherein a latent image is formed by a latent image forming method in which an image exposed portion serves as a coloring portion, and an inverting development method is employed, i.e., toner charged in the same polarity as a potential of a non-exposed portion is applied to the latent image, thereby performing development.
In the case of FIG. 2, uniform charging is performed on the surface of an image carrier in the same initial state as shown in FIG. 1. Then, first image exposure is performed on the charged surface by an exposure means such as a laser beam scanner based on color image data of the first color so that the coloring portion has a potential of substantially 0 V. The obtained latent image is then subjected to first development using a predetermined color toner (in this case, unlike in FIG. 1, toner charged in the same polarity as that of the image carrier). Thereafter, image exposure is performed by the same or different exposure means based on color image data of the second color. A portion of the obtained latent image at a potential of 0 V is developed by the corresponding color toner. Third and fourth latent image formation and development operations are performed to obtain a multicolor toner image. In this case, even if a toner T charged in the same polarity as that of the image carrier is applied to the latent image having a potential of substantially 0 V, the potential of the latent image does not become substantially equal to a background potential, as shown in FIG. 2. During development wherein a toner T' of a different color is applied to the latent image formed later, the toner T' tends to become attached to the latent image portion on which the toner T has already been attached although exposure, i.e., a write operation is not performed. Therefore, by utilizing a feature that color toners tend to be overlaid on each other without being mixed, a single-color image or a multicolor image with high sharpness can be obtained.
In the case of FIG. 2, a toner image is formed to positively overlay a position to which toner has already been attached. However, in the case of FIG. 3, in order to prevent color mixture caused by nonselective attachment of different color toners during the following development processes, re-charging is performed after the first development to smooth the surface potential. In FIG. 3, the operations from initialization to the first development are the same as those in FIG. 2. Thereafter, unlike in FIG. 2, the image carrier is uniformly charged, and second image exposure and second development are performed on the charged surface. Similarly, third and fourth latent image formation and development operations are performed. In this manner, in FIG. 3 wherein after the first development, the surface of the image carrier is uniformly re-charged to perform the following latent image formation and development, a position to which the toner image has already been formed can be exposed to again form a latent image thereon. In addition, unless an image position to which the toner has already been attached is exposed, the following different color toners will not be easily attached thereto.
In the case of FIG. 4, different color toners are especially prevented from being attached to an image position to which the toner has already been attached. In FIG. 4, the operations from initialization to the first development are the same as those in FIG. 3. After the first development, the surface of the image carrier is uniformly exposed using an exposure lamp, and second charging is then performed. Alternatively, the surface of the image carrier is uniformly charged, and then second image exposure and second development are performed. Similarly, third and fourth latent image formation and development operations are repeated. After development, if the surface of the image carrier is uniformly charged first, the potential of the image carrier including a portion which was developed by toner becomes substantially 0. When second charging is then performed, a difference between the potential of the portion applied with toner and that of other portions is decreased, and the surface of the image carrier can be uniformly charged. This also leads to a preferred result for a photosensitive body having an optical memory. When second charging is performed after development to uniformly charge the surface of the image carrier, and then uniform weak exposure is performed, the charged state of the surface of the image carrier to which the toner is attached has a higher potential than that in a case wherein no toner is attached. Therefore, when a latent image formed by next exposure is to be developed, since a portion to which the toner has already been attached has a potential equal to or higher than an unexposed portion unless it is exposed, attachment of toner to the portion can be effectively prevented.
In any process of FIGS. 1 to 4 described above, a toner image which has already been formed on the image carrier influences the second and subsequent image formation processes. This influence will be described in detail below:
(1) During Charging: PA0 Portions with and without toner have different potentials. PA0 (2) During Image Exposure: PA0 When exposure is performed on attached toner, exposure light is partially absorbed or reflected and scattered, an essential exposure amount is attenuated, and a latent image potential is increased. PA0 (3) During Development: PA0 A charge of attached toner or the thickness of a toner image influences development characteristics.
These factors are correlated with each other and give a complicated influence on image forming characteristics. For this reason, a color image signal must be corrected in correspondence with the image forming characteristics. Optical characteristics required for a toner image vary depending on whether an image on a transfer material on which the toner image formed is transferred is observed by reflection light like paper or is observed by transmitted light like a transparent film and an OHP film and on the characteristics of a spectral reflectance or spectral transmittance or these materials.
A color image is generally read to be separated into three colors, i.e., red (R), green (G), and blue (B), and the read color signals are converted into four colors, i.e., yellow (Y), magenta (M), cyan (C), and black (BK) corresponding to toner colors, thus performing image formation. More specifically, image data read as positive values R, G, and B are generally converted into positive values Y, M, C, and BK corresponding to attachment area ratios of respective color toners.
For this purpose, masking processing regarding toner characteristics is considered to be effective. In this processing, conversion from R, G, and B into Y, M, C, and BK is performed by a linear or quadratic (or higher) matrix arithmetic operation.
In a color laser printer or the like utilizing the above-mentioned image forming method, a pulse width of a write laser beam is changed so as to change a density of toner transferred to a transfer sheet. Thus, an amount of toner attached to an image carrier is changed by several steps, or a toner attachment area is stabilized using an area gradation method such as a Dither method, a density pattern method, or the like, thereby representing gradation.
When the above-mentioned methods are combined, a multicolor hard copy can be relatively easily obtained without using, e.g., a Dither pattern.
However, when a plurality of color toners are overlaid on the image carrier, the color toner which was developed first and the toner overlaid thereon have considerably different developing characteristics, as described above. For this reason, it is difficult to approximate the relationship between a write laser beam and a transfer amount, i.e., the relation of conversion from R, G, and B into Y, M, C, and BK by a polynomial.
It is very difficult to change a color correction method in accordance with the types of transfer material (i.e., paper, OHP, or others, and their spectral characteristics) and a user's favor.